A cement silo is a textbook permit required confined space. It has limited means of entry and exit, it is not designed for continuous occupancy, and it contains at least three hazards that have killed people doing this work: an atmosphere that can go oxygen deficient, material that can bridge overhead and collapse onto an entrant, and respirable crystalline silica.
If you are a producer buying silo cleaning, you are not just buying clean walls. You are putting a contractor's employee inside a space on your property, and the questions you ask before the work starts are the ones that matter. This is what a competent vendor should be able to answer without hesitating, and what the federal standard actually requires.
The governing standard
For general industry, permit required confined spaces are covered by OSHA 29 CFR 1910.146. It defines a confined space as one large enough to enter and perform work, with limited or restricted means of entry or exit, and not designed for continuous occupancy. It becomes permit required when it also contains or has the potential to contain a hazardous atmosphere, contains material with the potential to engulf an entrant, has an internal configuration that could trap or asphyxiate, or contains any other recognized serious hazard.
A cement silo checks several of those boxes at once, which is why the correct answer to "is this a permit space" is yes, before anyone looks.
The atmospheric numbers your vendor should know cold
Under 1910.146, an atmosphere is hazardous when, among other criteria, the oxygen concentration is below 19.5 percent or above 23.5 percent, or a flammable gas, vapor, or mist is present in excess of 10 percent of its lower flammable limit. Those are not best practice targets, they are the regulatory definition.
What that means on your pad: the space is tested before entry and monitored during it, and a reading outside those bounds is a stop, not a judgment call. Ask your vendor what their instrument is, when it was last calibrated and bump tested, and who records the readings. A crew that cannot answer the calibration question is a crew whose readings do not mean anything.
Silica is the hazard nobody asks about
Respirable crystalline silica is the exposure that does its damage quietly, over years, rather than in an incident report. OSHA's general industry standard, 29 CFR 1910.1053, sets a permissible exposure limit of 50 micrograms per cubic meter as an 8 hour time weighted average, with an action level of 25 micrograms per cubic meter, also an 8 hour TWA.
Silo cleaning generates airborne dust by design. Ask whether the contractor has an exposure assessment for this task, what respiratory protection the entrants wear, and whether that respirator selection was made against a measured exposure or a guess. This is the single most common gap in silo cleaning programs, because the consequence is invisible for a decade.
What the entry permit must contain
The permit is not paperwork theater. Under 1910.146(f) it has to identify the space and the purpose of entry, the date and authorized duration, the authorized entrants, the attendants, and the entry supervisor with a signature from whoever authorized the entry. It must list the hazards of the space and the isolation and control measures used against them, state the acceptable entry conditions, and record the initial and periodic test results with the tester's name and the time of each test.
It must also name the rescue and emergency services and how they are summoned, state the communication procedure between entrants and attendants, list the required equipment, and reference any additional permits such as a hot work permit.
Ask to see a completed permit from a recent job, with the identifying details redacted if the vendor prefers. A real program produces real permits. A program that exists only in a safety manual produces a blank form.
The attendant is not an extra pair of hands
This is where cutting corners usually happens, because an attendant looks like idle labor to someone watching the job from across the yard. Under 1910.146(i) the attendant has specific duties: know the hazards and their signs and symptoms, recognize behavioral effects of exposure in entrants, maintain an accurate count of who is inside, remain outside the space until relieved, maintain communication with entrants, and order evacuation when a prohibited condition arises or an entrant shows effects of exposure.
The attendant also summons rescue services, keeps unauthorized people away, and performs non entry rescue under the employer's procedure. Critically, the standard says the attendant performs no duties that would interfere with monitoring and protecting the entrants.
So if you walk past a silo job and the person at the opening is running a hose, moving equipment, or on an unrelated task, that job does not have an attendant. It has a second worker.
Isolation and lockout before anyone enters
A silo is connected to things that move and things that fill it. Aeration systems, bin vents, fill lines, screw conveyors, vibrators, and the pneumatic line from the tanker all need to be isolated and locked out before entry, and the lockout needs to be verified rather than assumed.
The failure mode here is specific and it has a body count: a truck arrives to fill a silo while a crew is inside it. Ask the vendor how they prevent that. The correct answer involves physical isolation of the fill line and communication with your own dispatch and receiving, not a sign on a gate.
Engulfment and bridged material
Stored cement and aggregate can bridge, forming a crust over a void, or build up on the walls in a rathole that releases without warning. An entrant standing on bridged material is standing on a trapdoor.
Competent practice is to work from above wherever possible, to never walk on stored product, to break bridges remotely rather than from underneath, and to use fall protection and retrieval systems rigged so that an entrant can be extracted without a second person entering. Ask what the retrieval setup is. A tripod and winch at the top opening is a different program than a rope tied to a handrail.
Rescue cannot be a phone call to 911 alone
The standard requires the employer to evaluate a prospective rescue service's ability to respond to a rescue summons in a timely manner, considering the hazards identified, and its proficiency with rescue related tasks and equipment. Most municipal departments are not equipped or trained for a silo rescue, and many will say so if asked directly.
The practical question for a producer is simple: if an entrant goes down inside the silo on your property, what happens in the next four minutes? If the answer is that somebody calls the fire department and waits, the rescue plan is a hope. Non entry retrieval, rigged and ready, is what turns it into a plan.
What to ask before you sign
Five questions separate a real program from a priced one. Who is the entry supervisor on my job and what is their training. Show me a completed permit from a recent silo. What gas monitor do you use and when was it last calibrated. What is the retrieval setup and who performs non entry rescue. What is your silica exposure assessment for this task and what respirator does it specify.
A vendor with a program answers all five in a few minutes. A vendor without one changes the subject to price.
How we run it
Express Chipping has been doing confined space work in silos and mixer drums since 1999, and we treat drum interiors as permit spaces for the same reasons we treat silos that way. Supplied air, harness, attendant at the opening, documented entry procedure, isolation verified before entry, and retrieval rigged before anyone goes in. That program is part of the service rather than a line item, because a vendor who prices safety separately has told you how optional they consider it.
See silo cleaning for how the work runs, Safety for our program, and concrete truck chipping for the drum side. To talk through a specific silo, call (562) 789-8058 or send a service request.
This article summarizes requirements from 29 CFR 1910.146 and 29 CFR 1910.1053 for orientation. The regulations themselves control, and your own program should be built with qualified safety counsel.